Method of Calibrating an Analyte-Measurement Device, and Associated Methods, Devices and Systems

ABSTRACT

The invention relates to a method for calibrating an analyte-measurement device that is used to evaluate a concentration of analyte in bodily fluid at or from a measurement site in a body. The method involves measuring a concentration, or calibration concentration, of an analyte in blood from an “off-finger” calibration site, and calibrating the analyte-measurement device based on that calibration concentration. The invention also relates to a device, system, or kit for measuring a concentration of an analyte in a body, which employs a calibration device for adjusting analyte concentration measured in bodily fluid based on an analyte concentration measured in blood from an “off-finger” calibration site.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 11/929,149 filed Oct. 30, 2007 of Benjamin J.Feldman, et al., which is a continuation of U.S. patent application Ser.No. 10/975,207 filed Oct. 27, 2004, now U.S. Pat. No. 7,299,082, whichis related to, and claims priority based on, U.S. Patent Application No.60/516,599 of Feldman et al. (hereinafter, the “Feldman et al.Application”) filed on Oct. 31, 2003, which is the subject of Feldman etal., A Continuous Glucose Sensor Based on Wired EnzymeTechnology-Results from a 3-Day Trial in Patients with Type I Diabetes,Diabetes Technology & Therapeutics, Vol. 5, No. 5, pp. 769-779 (2003)(hereinafter, the “Feldman et al. Publication”). This application isalso related to U.S. Pat. No. 6,881,551, which issued on Apr. 19, 2005;U.S. Pat. No. 6,551,494, which issued on Apr. 22, 2003; U.S. Pat. No.6,514,718, which issued on Feb. 4, 2003; U.S. Pat. No. 6,175,752, whichissued on Jan. 16, 2001; and U.S. Pat. No. 6,565,509, which issued onMay 20, 2003. Each of the aforementioned applications, publications, andpatents are incorporated herein in their entirety and for all purposesby this reference.

TECHNICAL FIELD

The invention relates to the calibration of an analyte-measurementdevice adapted to determine the concentration of an analyte in a fluidfrom a measurement site within a body, such as an animal body, amammalian body, or a human body. The invention further relates to theuse of a calibration standard that is based on a concentration of ananalyte in blood from a calibration site that is not accessed through asurface of a fingertip, or is not accessed through a surface of thefinger, or is not on or within a finger. The invention is particularlysuited for calibrating partially or fully implantable glucose-monitoringdevices, such as transcutaneous or subcutaneous glucose-monitoringdevices. Devices, systems and kits making use of the aforementionedmethod are provided as well.

BACKGROUND

There are a number of instances when it is desirable or necessary tomonitor the concentration of an analyte, such as glucose, lactate, oroxygen, for example, in a fluid of a body, such as a body of an animal.The animal may be a mammal, such as a human, by way of example. Forexample, it may be desirable to monitor the level of various analytes inbodily fluid, such as blood, that may have detrimental effects on abody.

In a particular example, it may be desirable to monitor high or lowlevels of glucose in blood that may be detrimental to a human. In ahealthy human, the concentration of glucose in the blood is maintainedbetween about 0.8 and about 1.2 mg/mL by a variety of hormones, such asinsulin and glucagons, for example. If the blood glucose level is raisedabove its normal level, hyperglycemia develops and attendant symptomsmay result. If the blood glucose concentration falls below its normallevel, hypoglycemia develops and attendant symptoms, such asneurological and other symptoms, may result. Both hyperglycemia andhypoglycemia may result in death if untreated. Maintaining blood glucoseat an appropriate concentration is thus a desirable or necessary part oftreating a person who is physiologically unable to do so unaided, suchas a person who is afflicted with diabetes mellitus.

Certain compounds may be administered to increase or decrease theconcentration of blood glucose in a body. By way of example, insulin canbe administered to a person in a variety of ways, such as throughinjection, for example, to decrease that person's blood glucoseconcentration. Further by way of example, glucose may be administered toa person in a variety of ways, such as directly, through injection oradministration of an intravenous solution, for example, or indirectly,through ingestion of certain foods or drinks, for example, to increasethat person's blood glucose level.

Regardless of the type of adjustment used, it is typically desirable ornecessary to determine a person's blood glucose concentration beforemaking an appropriate adjustment. Typically, blood glucose concentrationis monitored by a person or sometimes by a physician using an in vitrotest that requires a blood sample that is relatively large in volume,such as three microliters (μL) or more. The person may obtain the bloodsample by withdrawing blood from a blood source in his or her body, suchas a vein, using a needle and syringe, for example, or by lancing aportion of his or her skin, using a lancing device, for example, to makeblood available external to the skin, to obtain the necessary samplevolume for in vitro testing. (See U.S. Provisional Patent ApplicationNo. 60/424,414 of Saikley et al. filed on Nov. 6, 2002; and U.S. PatentApplication Publication No. 2004/0138588 A1 of Saikley et al. filed onNov. 4, 2003.) The person may then apply the fresh blood sample to atest strip, whereupon suitable detection methods, such as calorimetric,electrochemical, or photometric detection methods, for example, may beused to determine the person's actual blood glucose level. The foregoingprocedure provides a blood glucose concentration for a particular ordiscrete point in time, and thus, must be repeated periodically, inorder to monitor blood glucose over a longer period.

Since the tissue of the fingertip is highly perfused with blood vessels,a “finger stick” is generally performed to extract an adequate volume ofblood for in vitro glucose testing. By way of example, a finger stickmay involve lancing the fingertip and “milking” the adjacent tissue,such that an adequate volume of blood is available on the fingertipsurface. Unfortunately, the fingertip is also densely supplied with painreceptors, which can lead to significant discomfort during the bloodextraction process. Thus, conventional extraction procedures aregenerally inconvenient and often painful for the individual,particularly when frequent samples are required.

A less painful method for obtaining a blood sample for in vitro testinginvolves lancing an area of the body having a lower nerve ending densitythan the fingertip, such as the hand, the arm, or the thigh, forexample. Such areas are typically less supplied, or not heavilysupplied, with near-surface capillary vessels, and thus, blood. Forexample, a total blood flow of 33±10 mL/100 gm-min at 20° C. has beenreported for fingertips, while a much lower total blood flow of 6 to 9mL/100 gm-min has been reported for forearm, leg, and abdominal skin.(See: Johnson, Peripheral Circulation, John Wiley & Sons, p. 198(1978).) As such, lancing the body in these regions typically producessub-microliter samples of blood that are not sufficient for most invitro blood glucose-monitoring systems.

Glucose-monitoring systems that allow for sample extraction from sitesother than the finger and that can operate using small samples of blood,have been developed. For example, U.S. Pat. No. 6,120,676 to Heller etal. describes devices that permit generally accurate electrochemicalanalysis of an analyte, such as glucose, in a small sample volume ofblood. Typically, less than about one μL of sample is required for theproper operation of these devices, which enables glucose testing through“arm sticks” rather than finger sticks. Additionally, commercialproducts for measuring glucose levels in blood that is extracted fromsites other than the finger have been introduced, such as the FreeStyle®blood glucose-monitoring system (Abbott Diabetes Care, formerly known asTheraSense, Inc., Alameda, Calif.) that is based on the above-referencedU.S. Pat. No. 6,120,676.

However, differences between the circulatory physiology of finger sitesand “off-finger” sites have led to differences in the measurements ofblood glucose levels associated with those different sites, as reportedin McGarraugh et al., Glucose Measurements Using Blood Extracted fromthe Forearm and the Finger, TheraSense, Inc., Alameda, Calif. (2001),and McGarraugh et al., Physiological Influences on Off-Finger GlucoseTesting, Diabetes Technology & Therapeutics, Vol. 3, No. 3, pp. 367-376(2001). The former study indicates that stimulating blood flow at theskin surface of the arm may reduce these differences in certaincircumstances when the off-finger site is the arm. In the latter study,the differences between blood glucose measurements using capillary bloodfrom the finger and those using capillary blood from the arm wereattributed to a time lag in the glucose response on the arm with respectto the glucose response on the finger that was observed when the glucoseconcentration was changing. This time lag varied from subject-to-subjectin a range of five to twenty minutes. The study found that when glucoseconcentration is decreasing rapidly into a state of hypoglycemia, thistime lag could delay the detection of hypoglycemia. Thus, it wasdetermined that relative to the arm, the finger was a preferable testsite for testing for hypoglycemia.

It follows that while it may be desirable to move away from the fingeras a site for obtaining blood samples for discrete or periodic in vitroblood glucose determinations, in view of the pain involved, for example,it has not heretofore been deemed practical to do so to effectivelymonitor for low blood glucose levels that may be detrimental to anindividual.

In addition to the discrete or periodic, in vitro, bloodglucose-monitoring systems described above, at least partiallyimplantable, or in vivo, blood glucose-monitoring systems, which aredesigned to provide continuous in vivo measurement of an individual'sblood glucose concentration, have been described. (See, e.g., U.S. Pat.Nos. 6,248,067 to Causey et al.; 6,212,416 to Ward et al.; 6,175,752 toSay et al.; 6,119,028 to Schulman et al.; 6,091,979 to Pfeiffer et al.;6,049,727 to Crothall et al.; and 5,791,344 to Schulman et al.; andInternational Publication No. WO 00/78992.) Although optical means ordevices may be employed to monitor glucose concentration, a number ofthese in vivo systems are based on “enzyme electrode” technology,whereby an enzymatic reaction involving glucose oxidase is combined withan electrochemical sensor for the determination of an individual's bloodglucose level. By way of example, the electrochemical sensor may beinserted into a blood source, such as a vein or other blood vessel, forexample, such that the sensor is in continuous contact with blood andcan effectively monitor blood glucose levels. Further by way of example,the electrochemical sensor may be placed in substantially continuouscontact with bodily fluid other than blood, such as dermal orsubcutaneous fluid, for example, for effective monitoring of glucoselevels in such bodily fluid. Relative to discrete or periodicmonitoring, continuous monitoring is generally more desirable in that itmay provide a more comprehensive assessment of glucose levels and moreuseful information, such as predictive trend information, for example.Subcutaneous continuous glucose monitoring is also desirable for anumber of reasons, one being that continuous glucose monitoring insubcutaneous bodily fluid is typically less invasive than continuousglucose monitoring in blood.

While continuous glucose monitoring is desirable, there are severaldrawbacks associated with the manufacture and calibration of continuousglucose-monitoring devices. By way of example, based on currentmanufacturing techniques, it may be impossible to account forsensor-to-sensor or subject-to-subject variability in performingaccurate factory calibration. Further by way of example,individual-specific calibration may be desirable or required to accountfor subject-to-subject variability, such as subject-to-subjectphysiological variability. If an individual-specific calibration iscalled for, a sample of the individual's blood may be required in orderto calibrate a glucose monitor for that individual's use.

Further development of calibration methods, as well asanalyte-monitoring devices, systems, or kits employing same, isdesirable.

SUMMARY OF THE INVENTION

The concentration of a specific analyte at one area of a body may varyfrom that at another area. Herein, a body refers to a body of an animal,such as a mammal, and includes a human. Such a variation may beassociated with a variation in analyte metabolism, production, and/ortransportion from one area of the body and another. When data obtainedfrom one area of the body is used to calibrate an analyte-measurement ormonitoring device for a particular individual, such a variation mayresult in improper calibration of the device for that individual.According to one aspect of the present invention, a method ofcalibrating such a device that accounts for such a variation, isprovided.

For example, one aspect of the invention relates to a method forcalibrating an analyte-measurement device that is adapted to evaluatethe analyte concentration in a bodily fluid from a specific measurementsite in a body. The method involves determining the concentration of theanalyte in blood from a calibration site within the body that is notaccessed through a surface of a fingertip, and, based on thatdetermination, calibrating the analyte-measurement device. Preferably,the calibration site is not accessed through a surface of a finger. Mostpreferably, the calibration site is not on or within a finger. By way ofexample, but not limitation, the calibration site may be accessedthrough a surface of a palm, a hand, an arm, a thigh, a leg, a torso, oran abdomen, of the body, and may be located within a palm, a hand, anarm, a thigh, a leg, a torso, or an abdomen, of the body. An in vitroblood glucose-monitoring device, such as the above-mentioned FreeStyle®blood glucose-monitoring device, may be used for determining theconcentration of the analyte in the blood from the calibration site, oran in vivo measurement device or sensor may be used. Theanalyte-measurement device undergoing calibration may be, and preferablyis, an in vivo glucose-monitoring device, such as that described in U.S.Pat. No. 6,175,752 of Say et al. filed on Apr. 30, 1998, U.S. Pat. No.6,329,161 of Heller et al. filed on Sep. 22, 2000, U.S. Pat. No.6,560,471 of Heller et al. filed on Jan. 2, 2001, U.S. Pat. No.6,579,690 of Bonnecaze et al. filed on Jul. 24, 2000, U.S. Pat. No.6,654,625 of Say et al. filed on Jun. 16, 2000, and U.S. Pat. No.6,514,718 of Heller et al. filed on Nov. 29, 2001, for example. It iscontemplated that the analyte-measurement device may be an in vivoFreeStyle Navigator® glucose monitoring device (Abbott Diabetes CareInc.), based on the foregoing U.S. Pat. Nos. 6,175,752, 6,329,161,6,560,471, 6,579,690, 6,654,625, and 6,514,718, that is currently inclinical trials, though not now commercially available.

Another aspect of the invention relates to a method for monitoring theconcentration of an analyte in a body. The method involves determining aconcentration of the analyte in blood from a calibration site, such asthat described above; inserting a sensor into the body at a specificmeasurement site; obtaining at least two signals indicative of theconcentration of the analyte in the bodily fluid at that measurementsite via the sensor; and adjusting those signals based on theconcentration of the analyte in blood from the calibration site. An invitro blood glucose-monitoring device, such as the above-mentionedFreeStyle® blood glucose-monitoring device, may be used for determiningthe concentration of the analyte in the blood from the calibration site,although in vivo measurement devices or sensors may also be used. Thesensor is chosen as one that is sufficient for determining theconcentration of the analyte in the bodily fluid at the measurementsite, or providing a signal indicative of such analyte concentration,such as that associated with an in vivo glucose monitoring device, asdescribed above. Preferably, the sensor is exposed to the bodily fluidin a thorough or substantially continuous manner. Preferably, obtainingthe signals indicative of the concentration of the analyte in the bodilyfluid at the measurement site occurs over a period of time, such as fromabout one day to about three days or more, for example.

According to yet another aspect of the invention, a surface of the bodyadjacent to the calibration site may be rubbed prior to thedetermination of analyte concentration in blood from the calibrationsite. Preferably, the rubbing is sufficient to enhance mobility of fluidat the calibration site. Typically, manually rubbing the surface of anarm, leg, or abdomen, for example, with a comfortable or moderate amountof pressure for a few seconds, up to a minute or more, will suffice toenhance mobility of fluid at a nearby calibration site within the arm,leg, or abdomen, respectively. Rubbing pressure and time can be variedappropriately, for example, less pressure can be applied for longer, andmore pressure can be applied more briefly, and either or both can bevaried as desirable or necessary for a particular calibration site. Anyappropriate means or devices, manual or otherwise, may be used to rubthe surface or to enhance mobility of the fluid at the calibration site.

A method according to the present invention is well suited for use inconnection with a device that allows for the self-monitoring of glucoselevels. Such a method may involve determining or measuring an analyteconcentration in subcutaneous fluid, or in dermal fluid, or ininterstitial fluid, for example. Any of the above-described methods mayutilize any of a number of calibration sites in a body, such as those inthe arms, the legs, the torso, the abdomen, or any combination thereof,merely by way of example. In humans, arms and legs are particularlyconvenient calibration sites. The measurement and calibration sites maybe located in different parts of a body, or in the same region orregions of the body. The same or different types of devices may be usedto measure analyte concentration in the bodily fluid and in the blood.Depending on the particular physiological conditions of the calibrationsite or sites, it may be desirable to rub a surface of the body adjacentthe calibration site, such as arm skin that is above or near acalibration site within an arm, as previously described. (See: U.S. Pat.No. 6,591,125 of Buse et al. filed on Jun. 27, 2000.)

According to yet another aspect of the present invention, a system orkit for measuring the concentration of an analyte in a body is provided.The system comprises a measurement sensor for providing a signalindicative of a concentration of the analyte in the bodily fluid at themeasurement site, a calibration sensor for determining a concentrationof the analyte in blood from the calibration site, and a calibrationdevice in operative communication with the measurement sensor and thecalibration sensor for receiving data therefrom. The measurement sensormay be a disposable device, and may be independent, separate, separableor detachable relative to the calibration device, and may be wirelesslyor physically associated with the calibration device when in use.Appropriate measurement sensors include the various in vivo measurementdevices or sensors described above. The calibration sensor may be anysensor sufficient for determining the concentration of the analyte inblood at the calibration site. Appropriate calibration sensors includethe various in vitro measurement devices or sensors described above,although in vivo measurement devices or sensors may also be used. Thecalibration device comprises a receiving element for receiving at leastone signal obtained via the measurement sensor, a receiving element forreceiving at least one concentration value obtained via the calibrationsensor, and calibration element for calibrating the signal obtained viathe measurement sensor based on the value obtained via the calibrationsensor. The receiving element may comprise a storage element for storingany value received. The calibration element may comprise an algorithmfor making the calibration or adjustment, which algorithm may beembodied in software.

Preferably, the measurement sensor is sufficient for electrochemicallydetermining the concentration of the analyte in the bodily fluid. Whenan electrochemical measurement sensor is used, the sensor generallycomprises a working electrode and a counter electrode. When the analyteof interest is glucose, the working electrode generally comprises aglucose-responsive enzyme and a redox mediator. The redox mediator maycomprise an osmium (Os)- or a ruthenium (Ru)-containing complex, by wayof example, preferably, the former. Preferably, the redox mediator isnon-leachable relative to the working electrode, such that it does notleach from the working electrode into the body over the lifetime of thesensor. Most preferably, the redox mediator is immobilized on theworking electrode.

Preferably, the calibration sensor is sufficient for electrochemicallydetermining the concentration of the analyte in blood based on anysuitable volume of blood. While this volume may be about 3 μL for somemeasurement sensors, as described above, it is preferably less than orequal to about 1 μL of blood, more preferably, less than or equal toabout 0.5 μL of blood, and still more preferably, less than or equal toabout 0.2 μL of blood, such as the smallest amount sufficient for ameaningful measurement. The calibration sensor may be an in vitroelectrochemical sensor, as described above, or an in vivoelectrochemical sensor, as also described above, designed for sensing inblood, typically and preferably the former.

These and various other aspects, features and embodiments of the presentinvention are further described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of various aspects, features and embodiments ofthe present invention is provided herein with reference to theaccompanying drawings, which are briefly described below. The drawingsare illustrative and are not necessarily drawn to scale. The drawingsillustrate various aspects or features of the present invention and mayillustrate one or more embodiment(s) or example(s) of the presentinvention in whole or in part. A reference numeral, letter, and/orsymbol that is used in one drawing to refer to a particular element orfeature may be used in another drawing to refer to a like element orfeature.

Each of FIG. 1A (FIG. 1A) and FIG. 1B is a schematic illustration of asystem or portions thereof for measuring a concentrate of an analyte ina bodily fluid that may be employed, according to various aspects of thepresent invention. These two figures may be collectively referred to asFIG. 1 (FIG. 1) herein.

FIG. 2A (FIG. 2A), FIG. 2B (FIG. 2B), and FIG. 2C (FIG. 2C),collectively and sequentially illustrate a calibration process oralgorithm that may be employed, according to various aspects of thepresent invention. These three figures may be collectively referred toas FIG. 2 (FIG. 2) herein.

FIG. 3 (FIG. 3) is a schematic illustration of an analyte-measuring ormonitoring device, a portion of which is enlarged for illustrationpurposes, that may be employed, according to various aspects of thepresent invention.

FIG. 4A (FIG. 4A) is a schematic illustration of a sensing layer that isassociated with a working electrode of an analyte-measuring ormonitoring device, such as that illustrated in FIG. 3. FIG. 4B (FIG. 4B)is an illustration of the structure of a redox polymer component of asensing layer, such as that illustrated in FIG. 4A. FIGS. 4A and 4B maybe collectively referred to as FIG. 4 (FIG. 4) herein.

FIG. 5 (FIG. 5) is a overlay plot of representative data (−) from anabdominally implanted analyte-measuring or monitoring device in raw,uncalibrated current (nA) on the left axis versus time (days) and venousplasma data (Δ) in glucose concentration (mg/dL) on the right axisversus time (days), according to an Experimental Study described herein.

FIG. 6 (FIG. 6) is a plot of representative data (−) from anarm-implanted analyte-measuring or monitoring device, as calibrated,venous plasma data (Δ), and arm-capillary blood data (□), in glucoseconcentration (mg/dL) versus time (days), according to an ExperimentalStudy described herein.

FIG. 7 (FIG. 7) is a plot of representative data (−) from anarm-implanted analyte-measuring or monitoring device, as calibrated,representative data (−) from an abdomen-implanted analyte-measuring ormonitoring device, as calibrated, and venous plasma data (Δ), in glucoseconcentration (mg/dL) versus time (days), according to an ExperimentalStudy described herein.

FIG. 8 (FIG. 8) is a plot of glucose concentration data (mg/dL) fromarm- or abdomen-implanted analyte-measuring or monitoring devices, ascalibrated, versus that data from venous blood, in the form of a Clarkeerror grid, according to an Experimental Study described herein.

DETAILED DESCRIPTION OF THE INVENTION

In the description of the invention herein, it will be understood that aword appearing in the singular encompasses its plural counterpart, and aword appearing in the plural encompasses its singular counterpart,unless implicitly or explicitly understood or stated otherwise. Merelyby way of example, reference to “an” or “the” “analyte” encompasses asingle analyte, as well as a combination and/or mixture of two or moredifferent analytes, reference to “a” or “the” “concentration value”encompasses a single concentration value, as well as two or moreconcentration values, and the like, unless implicitly or explicitlyunderstood or stated otherwise. Further, it will be understood that forany given component described herein, any of the possible candidates oralternatives listed for that component, may generally be usedindividually or in combination with one another, unless implicitly orexplicitly understood or stated otherwise. Additionally, it will beunderstood that any list of such candidates or alternatives, is merelyillustrative, not limiting, unless implicitly or explicitly understoodor stated otherwise.

Various terms are described below to facilitate an understanding of theinvention. It will be understood that a corresponding description ofthese various terms applies to corresponding linguistic or grammaticalvariations or forms of these various terms. It will also be understoodthat the invention is not limited to the terminology used herein, or thedescriptions thereof, for the description of particular embodiments.Merely by way of example, the invention is not limited to particularanalytes, bodily or tissue fluids, blood or capillary blood, or sensordesigns or usages, unless implicitly or explicitly understood or statedotherwise, as such may vary.

The terms “amperometry” and “amperometrically” refer to the measurementof the strength of a current and include steady-state amperometry,chronoamperometry, and Cottrell-type measurements.

The term “bodily fluid” in the context of the invention encompasses allnon-blood bodily fluid that can be found in the soft tissue of anindividual's body, such as subcutaneous, dermal, or interstitial tissue,in which the analyte may be measured. By way of example, the term“bodily fluid” encompasses a fluid such as dermal, subcutaneous, orinterstitial fluid.

The term “blood” in the context of the invention encompasses whole bloodand its cell-free components, such as plasma and serum. The term“capillary blood” refers to blood that is associated with anyblood-carrying capillary of the body.

The term “concentration” may refer to a signal that is indicative of aconcentration of an analyte in a medium, such as a current signal, forexample, to a more typical indication of a concentration of an analytein a medium, such as mass of the analyte per unit volume of the medium,for example, or the like.

“Coulometry” refers to the determination of charge passed or projectedto pass during complete or nearly complete electrolysis of a compound,either directly on the electrode or through one or moreelectron-transfer agents. The charge is determined by measurement ofelectrical charge passed during partial or nearly complete electrolysisof the compound or, more often, by multiple measurements during theelectrolysis of a decaying current over an elapsed period. The decayingcurrent results from the decline in the concentration of theelectrolyzed species caused by the electrolysis.

A “counter electrode” refers to one or more electrodes paired with theworking electrode, through which passes an electrochemical current equalin magnitude and opposite in sign to the current passed through theworking electrode. The term “counter electrode” is meant to includecounter electrodes that also function as reference electrodes (i.e., acounter/reference electrode) unless the description provides that a“counter electrode” excludes a reference or counter/reference electrode.

The term “electrolysis” refers the electrooxidation or electroreductionof a compound either directly at an electrode or via one or moreelectron-transfer agents, such as redox mediators and/or enzymes, forexample.

An “immobilized” material refers to a material that is entrapped on asurface or chemically bound to a surface.

An “implantable” device refers to a fully implantable device that isimplanted fully within a body and/or an at least partially implantabledevice that is at least partially implanted within a body. An example ofan at least partially implantable sensing device is a transcutaneoussensing device, sometimes referred to as a subcutaneous sensing device,that is associated with a portion that lies outside of a body and aportion that penetrates the skin from the outside of the body andthereby enters the inside of the body.

The term “measure,” as in “to measure the concentration,” is used hereinin its ordinary sense and refers to the act of obtaining an indicator,such as a signal, that may be associated with a value, such asconcentration, for example, and to the act of ascertaining a value, suchas a concentration, for example. The term “monitor,” as in “to monitorthe concentration,” refers to the act of keeping track of more than onemeasurement over time, which may be carried out on a systematic,regular, substantially continuous, and/or on-going basis. The termsmeasure and monitor may be used generally herein, such as alternately,alternatively, or interchangeably, or more specifically, as justdescribed.

The term “measurement” may refer to a signal that is indicative of aconcentration of an analyte in a medium, such as a current signal, forexample, to a more typical indication of a concentration of an analytein a medium, such as mass of the analyte per unit volume of the medium,for example, or the like. The term “value” may sometimes be used hereinas a term that encompasses the term “measurement.”

The term “patient” refers to a living animal, and thus encompasses aliving mammal and a living human, for example. The term “subject” maysometimes be used herein as a term that encompasses the term “patient.”

The term “redox mediator” refers to an electron-transfer agent thattransfers electrons between a compound and a working electrode, eitherdirectly or indirectly.

The term “reference electrode” encompasses a reference electrode thatalso functions as a counter electrode (i.e., a counter/referenceelectrode), unless the description provides that a “reference electrode”excludes a counter/reference electrode.

The term “working electrode” refers to an electrode at which a candidatecompound is electrooxidized or electroreduced with or without the agencyof a redox mediator.

The invention generally relates to the calibration of a device adaptedto measure or monitor a concentration of an analyte in a body. Theinvention exploits a correspondence that exists between a concentrationof an analyte found in a bodily fluid of an individual and aconcentration of the same analyte found in blood of that individual. Forexample, according to the present invention, a concentration of ananalyte in blood from a particular calibration site within the body ofan individual is used to calibrate a device that is adapted to measureor monitor a concentration of the analyte at a measurement site in thebody of that individual.

As previously described, it is often undesirable or painful to obtainblood from a fingertip or finger. The calibration method of the presentinvention does not demand this. That is, according to the presentinvention, a calibration site may be selected as one that is notaccessed from a surface of a fingertip, one that is not accessed from asurface of a finger, or one that is not on or within a finger,preferably the latter. Merely by way of convenience, but not limitation,such a calibration site may be referred to as an “off-finger”calibration site. By way of example, but not limitation, the calibrationsite may be accessed through a surface of a palm, a hand, an arm, athigh, a leg, or an abdomen, of the body, and may be located within apalm, a hand, an arm, a thigh, a leg, or an abdomen, of the body, or anyother bodily site wherein the blood or capillary blood at the sitegenerally tracks bodily fluid in terms of glucose concentration. Theoff-finger calibration site is typically located up to about 2 mmbeneath the exterior surface of the epidermis, or up to the maximumdepth appropriate for a “stick” by a lancet or other appropriate meansor device.

As previously described, there are a number of different systems thatcan be used in the measuring or monitoring of glucose levels in a body,including those that comprise a glucose sensor that is adapted forinsertion into a subcutaneous site within the body for the continuousmonitoring of glucose levels in bodily fluid of the subcutaneous site.For example, U.S. Pat. No. 6,175,752 to Say et al. employs such a sensorthat comprises at least one working electrode that is associated with aredox enzyme, wherein the redox enzyme is sufficient to catalyze areaction that is associated with the detection of glucose. This sensorfurther comprises a counter electrode and a reference electrode, or acombined counter/reference electrode, and may further comprise atemperature probe. Such a sensor is further described in theExperimental Study below.

A suitable sensor may work as now described. The sensor is placed,transcutaneously, for example, into a subcutaneous site such thatsubcutaneous fluid of the site comes into contact with the sensor. Thesensor operates to electrolyze an analyte of interest in thesubcutaneous fluid such that a current is generated between the workingelectrode and the counter electrode. A value for the current associatedwith the working electrode is determined periodically. If multipleworking electrodes are used, current values from each of the workingelectrodes may be determined periodically. A microprocessor may be usedto collect these periodically determined current values or to furtherprocess these values.

The periodically determined current values may be processed in variousways. By way of example, current values may be checked to determinewhether they are within a predetermined range. If the current values arewithin the predetermined range, one of the current values is convertedto an analyte concentration by way of a calibration. Further by way ofexample, in the case of multiple working electrodes, current values fromeach of the working electrodes may be compared to determine whether theydiffer by a predetermined threshold amount. If the current values arewithin the predetermined range and do not differ by more than thepredetermined threshold amount, one of the current values is convertedto an analyte concentration by way of a calibration. Sensor-specificcalibration may be performed during the manufacture of the sensor, asdescribed elsewhere herein. Alternative or additionalindividual-specific calibration may be performed on an individual basis,as also described herein. Further calibration may be needed when thecurrent values from a working electrode or from each of multiple workingelectrodes are not within the predetermined range, or when the currentvalues from each of multiple working electrodes differ by more than thepredetermined threshold amount. If the current values do not meet one ormore of the established criteria, none of the current values may beacceptable for conversion into an analyte concentration. An indication,such as a code, may be displayed or otherwise transmitted, such as viaaudio, visual, vibrational, sensory, or other suitable notificationmeans or device, to indicate this fact. If analyte concentration issuccessfully determined, it may be displayed, stored, and/or otherwiseprocessed to provide useful information. By way of example, analyteconcentrations may be used as a basis for determining a rate of changein analyte concentration, which should not change at a rate greater thana predetermined threshold amount. If the rate of change of analyteconcentration exceeds the predefined threshold, an indication may bedisplayed or otherwise transmitted to indicate this fact.

The sensor may have undergone calibration during the manufacturingprocess. However, as previously described, such calibration may beinsufficient in terms of accounting for sensor-to-sensor orsubject-to-subject variability. Thus, individual-specific calibrationmay be desirable or required to account for subject-to-subjectvariability, such as subject-to-subject physiological variability. Insuch a calibration, a sample of blood may be extracted from acalibration site within the individual and measured to obtain a glucoseconcentration for use as a calibration point. The measurement may becarried out using any of various known means, devices and methods, suchas via the FreeStyle® blood glucose-monitoring system. The resultingglucose concentration can be entered into an analyte-monitoring deviceas a calibration code, as desirable or needed, for example, immediatelyafter sensor implantation or following notification of an invalidresult. The sensor may be calibrated manually, periodically, or asdesirable or necessary, during use.

As described above, blood samples are often obtained from sites withinhighly perfused areas of the body, such as sites within the fingertips.Blood-sampling from these sites is quite painful. Alternative sites,however, have not previously been thought to be sufficiently practicalor useful as sources for calibration samples. By way of example, in aprevious study, it was reported that capillary blood obtainedsimultaneously from different body sites have different glucoseconcentrations, and that the blood glucose levels obtained from the armand the finger were not perfectly correlated. (See: McGarraugh et al.,Glucose Measurements Using Blood Extracted from the Forearm and theFinger, TheraSense, Inc., Alameda, Calif. (2001); and McGarraugh et al.,Physiological Influences on Off-Finger Glucose Testing, DiabetesTechnology & Therapeutics, Vol. 3, No. 3, pp. 367-376 (2001).) Thus, ithas previously been thought that alternative sites are not suitable forblood-sampling for calibration purposes.

According to the present invention, blood-sampling at alternative sitesis used for calibration purposes. As demonstrated in the ExperimentalStudy described herein, the use of alternative sites for calibrationpurposes is advantageous for a number of reasons beyond pain reduction,such as allowing for the concentration of calibration points early on inthe period of use, allowing for the refinement of calibration asmultiple calibration points are obtained, allowing for the use ofreal-time data, and providing clinically accurate or acceptable results.

According to an embodiment of the present invention, a method forcalibrating a device sufficient for determining a concentration of ananalyte of interest at a measurement site within a body, comprisesproviding the device at the measurement site within the body,determining a concentration of the analyte in blood from an off-fingercalibration site within the body, and calibrating the device using theresulting analyte concentration. According to this method, the resultinganalyte concentration may serve as a baseline concentration of analytein the blood for calibration purposes. There is no particular limitationon the location of the measurement site. By way of example, anymeasurement site of practical utility may be used. Preferably, themeasurement site is also an off-finger measurement site, such as an arm,a leg, a torso, or an abdomen, for example. The measurement site istypically located up to about 8 mm beneath the exterior surface of theepidermis, preferably located from about 2 mm to about 6 mm beneath theexterior surface, and more preferably located from about 3 mm to about 5mm beneath the exterior surface.

According to another embodiment of the present invention, a method fordetermining a concentration of an analyte, such as glucose, in a bodilyfluid at a measurement site within a body, comprises inserting a device,such as those described herein, at the measurement site within the body,determining the concentration of an analyte of interest, such asglucose, in blood from an off-finger calibration site within a body, andcalibrating the device using the resulting analyte concentration. Inthis method, the sensor is used to determine at least two values for theconcentration of the analyte in the bodily fluid at the measuring site.Further, calibrating the device comprises adjusting the at least twovalues based on the concentration of the analyte in blood from thecalibration site. According to this method, the concentration of theanalyte in blood from the calibration site may be determine at leastonce, or at least twice, during the determination of the at least twovalues for the concentration of the analyte in the bodily fluid at themeasurement site. Here again, there is no particular limitation on thelocation of the measurement site, although preferably it is anoff-finger site, such as an arm, a leg, a torso, or an abdomen, forexample.

As demonstrated herein, the methods of the present invention areparticularly useful in connection with a device that is used to measureor monitor a glucose analyte, such as any such device described herein.These methods may also be used in connection with a device that is usedto measure or monitor another analyte, such as oxygen, carbon dioxide,proteins, drugs, or another moiety of interest, for example, or anycombination thereof, found in bodily fluid, such as subcutaneous fluid,dermal fluid (sweat, tears, and the like), interstitial fluid, or otherbodily fluid of interest, for example, or any combination thereof.Preferably, the device is in good contact, such as thorough andsubstantially continuous contact, with the bodily fluid.

According to yet another embodiment of the present invention, a systemor kit for measuring a concentration of an analyte in a bodily fluid ata measurement site within the body is provided. An example of such asystem 100 is schematically illustrated in FIG. 1A and FIG. 1B. Thesystem 100 comprises a measurement sensor 102, a calibration sensor 104,and a calibration device 106. The measurement sensor 102 is any suitablesensor that is sufficient for determining the concentration of theanalyte in the bodily fluid at a measurement site within the body, suchas any described herein. The calibration sensor 104 is any suitablesensor that is sufficient for determining a calibration concentration ofthe analyte in blood at an off-finger calibration site within the body.The location of the measurement sensor within the body is unrestricted,although some locations may be more desirable or practical, as describedabove. Preferably, the measurement site is an off-finger site.

The two sensors 102 and 104 may be completely independent, such as anindependent in vivo, continuous, glucose monitoring sensor and anindependent in vitro, discrete, glucose-testing strip, that arephysically separate, merely by way of example. The sensors 102 and 104may be provided in a system or kit 100 that comprises elementssufficient for calibration and use of the measurement sensor accordingto the present invention, such as the elements described below.

The measurement sensor 102 and the calibration device 106 may bephysically associated with one another, whether temporarily, detachably,or permanently. The measurement sensor 102 and the calibration device106 may be wirelessly associated, whether directly (not shown) orindirectly, as shown via transmission element 108 in FIG. 1A. Themeasurement sensor 102 may include a transmission element or device 108as a component (not shown), or may be operatively coupled to atransmission element or device 108, as shown in FIG. 1A and FIG. 1B. Thecoupling may be wireless or in the form of a direct physical connection,as shown in FIG. 1A, merely by way of example. The transmission elementor device 108 is of a construction sufficient for receiving a rawanalyte signal (represented by an encircled symbol) from the measurementsensor 102 and transmitting a raw analyte signal, such as a current, forexample, to the calibration element or device 106. The transmissiondevice 108 and the calibration device 106 are operatively coupled forcommunication therebetween. The coupling may be in the form of awireless connection, as shown in FIG. 1A, any other suitablecommunicative connection, or any combination thereof.

The calibration sensor 104 may include the calibration device 106 as acomponent (not shown), or may be operatively coupled to the calibrationdevice 106, as shown in FIG. 1A and FIG. 1B. The coupling may bewireless (not shown) or in the form of a direct physical connection, asshown in FIG. 1A, merely by way of example. Preferably, the calibrationdevice 106 is designed to receive calibration data from the calibrationsensor 104 automatically, rather than manually via the user, so as toreduce the chances of data entry error, for example.

As shown in FIG. 1B, the calibration device 106 comprises an element 110for receiving at least one signal or concentration value obtained viathe measurement sensor 102 and an element 112 for receiving at least oneconcentration value obtained via the calibration sensor 104, and acalibration element 114 for evaluating data, such as a signal or valuefrom the measurement sensor 112, and/or a value from the calibrationsensor 104. The receiving elements 110 and 112 may comprise any suitableelectronic circuitry, componentry, storage media, such as temporarystorage media or rewriteable storage media, a signal- or data-processingelement, a software element, or any combination thereof, merely by wayof example, and may be physically (wired, for example) or wirelesslyassociated with sensors 102 and 104, respectively. The calibrationelement 114 may comprise any suitable electronic circuitry, componentry,storage media, an algorithmic element, a data-processing element, asoftware element, or any combination thereof, for making the adjustmentor calibration. The calibration element 114 may comprise any suitablemeans or device for storing any suitable algorithm or software, such asany suitable storage media, for example, non-rewriteable electronicstorage media and/or read-only electronic storage media. As output 110,the calibration element 114 may provide an indication of operatingsensitivity 116, as shown in FIG. 1B, by way of example, for use inanother part of the system, such as a microprocessor (“μP”) 118, forcalibrating an analyte signal or value from the measurement sensor basedon the value from the calibration sensor, or calculating analyteconcentration. The calibration, or calculation of analyte concentration,may be found by dividing the raw analyte signal by the operatingsensitivity, when the sensitivity is expressed in appropriate units ofcurrent/concentration, such as nA/(mg/dL), for example. The system 100may further comprise any suitable communication means or device (notshown), operatively connected to the microprocessor 118, forcommunicating the analyte sensitivity to the user, to another system,and/or the like.

Preferably, the measurement sensor 102 is designed, constructed, orconfigured for ease in self-monitoring analyte concentration in bodilyfluid. Merely by way of example, the measurement sensor 102 may be anysuitable sensor described in U.S. Pat. No. 6,175,752 to Say et al. Themeasurement sensor 102 may be one suited for an in vitro measurement ofanalyte concentration in solution, or one suited for in vivo measurementof analyte concentration of a bodily fluid. Merely by way of example,the measurement sensor 102 may be one suited for partial or fullimplantation within a body, such as an in vivo sensor suited forcontinuous monitoring of an analyte concentration in a bodily fluid withthe body. The measurement sensor 102 may comprise ananalyte-diffusion-limiting membrane, as further described in relation tothe Experimental Study herein, although such a membrane is not required.(See: U.S. Pat. No. 6,932,894 of Mao et al. filed on May 14, 2002 (mayinclude a membrane); and U.S. Pat. No. 7,052,591 of Gao et al. filed onSep. 19, 2000 (may not include a membrane).)

According to a preferred embodiment of the present invention, themeasurement sensor 102 is one suited for electrochemical measurement ofanalyte concentration, and preferably, glucose concentration, in abodily fluid. In this embodiment, the measurement sensor 102 comprisesat least a working electrode and a counter electrode. It may furthercomprise a reference electrode, although this is optional. The workingelectrode typically comprises a glucose-responsive enzyme and a redoxmediator, as further described below in the Experimental Study, both ofwhich are agents or tools in the transduction of the analyte, andpreferably, glucose. Preferably, the redox mediator is non-leachablerelative to the working electrode. Merely by way of example, the redoxmediator may be, and preferably is, immobilized on the workingelectrode.

According to a most preferred embodiment of the present invention, themeasurement sensor 102 is one suited for in vivo, continuous,electrochemical measurement or monitoring of analyte concentration, andpreferably, glucose concentration, in a bodily fluid. In thisembodiment, the measurement sensor 102 is sufficiently biocompatible forits partial or full implantation within the body. By way of explanation,when an unnatural device is intended for use, particularly long-termuse, within the body of an individual, protective mechanisms of the bodyattempt to shield the body from the device. (See co-pending U.S.application Ser. No. 10/819,498 of Feldman et al. filed on Apr. 6, 2004,published as U.S. Publication No. 2005/0173245.) That is, such anunnatural device or portion thereof is more or less perceived by thebody as an unwanted, foreign object. Protective mechanisms of the bodymay encompass encapsulation of the device or a portion thereof, growthof tissue that isolate the device or a portion thereof, formation of ananalyte-impermeable barrier on and around the device or a portionthereof, and the like, merely by way of example. Encapsulation andbarrier formation around all or part of the implantable sensor maycompromise, significantly reduce, or substantially or completelyeliminate, the functionality of the device. Preferably, the measurementsensor 102 is sufficiently biocompatible to reduce, minimize, forestall,or avoid any such protective mechanism or its effects on the sensorfunctionality, or is associated with or adapted to incorporate amaterial suitable for promoting biocompatibility, such as asuperoxide-dismutase/catalase catalyst. (See co-pending U.S. applicationSer. No. 10/819,498 of Feldman et al. filed on Apr. 6, 2004.)Preferably, the measurement sensor 102 is sufficiently biocompatibleover the desired, intended, or useful life of the sensor.

It is also preferable that the measurement sensor 102 be relativelyinexpensive to manufacture and relatively small in size. It isparticularly preferable that the measurement sensor 102 be suitable forbeing treated as a disposable device, such that the measurement sensormay be disposed of and replaced by a new measurement sensor, forexample. As such, the measurement sensor 102 is preferably physicallyseparate from, or separable from, the calibration device 106 orcalibration sensor 104. A measurement sensor suitable for operating overa period of about 1 to 3 days, is desirable. A measurement sensorsuitable of operating over a longer period is contemplated, provided itprovides no significant ill effect in the body.

The calibration device 106 may comprise suitable electronic and othercomponents and circuitry such as those described in U.S. Pat. No.6,175,752 to Say et al. By way of example, the calibration device 106may comprise a potentiostat/coulometer suitable for use in connectionwith an electrochemical measurement sensor. The calibration device 106may be a device that is suitable for repeated or on-going use, even ifthe measurement sensor 102 is disposable. As such, the measurementsensor 102 and the calibration device 106 may be physically separate orcapable of physical separation or detachment.

According to embodiments of the present invention, the calibration sitemay be any off-finger site within a body that is a suitable source ofblood or capillary blood. Convenient calibration sites may be those thatare close to an exterior surface of the body. Preferred calibrationsites are those that have a sufficient supply of blood or capillaryblood for drawing a suitable sample and have a low density of painreceptors. Suitable calibration sites are located in an arm, a forearm,a leg, or a thigh, for example. Any suitable way or means of, or devicefor, measuring analyte concentration in blood or capillary blood at sucha calibration site, such as any of those described herein, iscontemplated as being of use according to the present invention.However, as obtaining a sufficient volume of blood for measurement maybe more difficult at an off-finger calibration site than at a fingertipor finger calibration site, a suitable way or means of, or device for,measuring analyte concentration in a small volume of blood or capillaryblood from an off-finger calibration site is preferred. A suitable wayor means or device may be any of those associated with a small volume,in vitro, analyte sensor, such as any of those described in U.S. Pat.No. 6,120,676 to Heller et al.; or any of those suitable for measuringanalyte concentration in preferably less than or equal to about 1 μL ofblood or capillary blood, more preferably, less than or equal to about0.5 μL of blood, and most preferably, less than or equal to about 0.2 μLof blood is used for calibration, such as any amount sufficient forobtaining a meaningful or useful measurement. In a preferred embodiment,such a way or means or device is electrochemical, such as amperometricor coulometric, for example.

According to embodiments of the present invention, the measurement sitemay be any site within a body that is a suitable source of bodily fluid.A suitable measurement sites is any such site that is suitable foroperation of the analyte-measurement or monitoring device. By way ofexample, suitable measurement sites include those in an abdomen, a leg,a thigh, an arm, an upper arm, or a shoulder, as described in U.S. Pat.No. 6,175,752 to Say et al. Preferably, the measurement site is in theupper arm or in the abdomen. The measurement site and the calibrationsites may be located in substantially the same region or part of thebody or in different regions or parts of a body.

The analyte-monitoring device may be calibrated a particular point or atvarious points in the analyte-monitoring process. The device istypically calibrated before it is used to monitor analyte concentrationin a body. As such, analyte concentration in blood or capillary bloodfrom the calibration site is typically measured within about fiveminutes to about one hour of sensor use or insertion within a body. Insome cases, it may be desirable or necessary to calibrate the deviceduring a period of analyte monitoring. As such, analyte concentration inblood or capillary blood may be measured once or more during such aperiod. Any suitable way or means of, or device for, measuring analyteconcentration in a bodily fluid at a measurement site may be used. Asuitable way or means or device may be electrochemical, as describedabove in connection with calibration measurements, albeit adapted asdesirable or necessary for the measurement of analyte concentration inthe bodily fluid rather than in blood.

Calibration may be described as a process by which a raw signal from ananalyte-measuring or monitoring sensor is converted into an analyteconcentration. By way of example, when an optical analyte sensor isused, the raw signal may be representative of absorbance, and when anelectrochemical analyte sensor is used, the raw signal may berepresentative of charge or current. Calibration may generally bedescribed in terms of three parts or phases, as described below.

In one phase, or a first phase, a calibration measurement may be madevia a calibration sensor and a raw signal may be gathered via an analytesensor more or less simultaneously. By more or less simultaneously, orsubstantially simultaneously, is meant within a period of up to about 10minutes; preferably, up to about 5 minutes; more preferably, up to about2 minutes; and most preferably, up to about 1 minute, in this context.In general, the calibration measurement is deemed or trusted as accuratebecause the performance of the calibration sensor has been verifiedthrough its own calibration process. Ideally, the calibrationmeasurement and the raw signal are obtained from identical samples.Practically, this is often not possible. In the latter case, therelationship between the calibration sample and the test sample must besufficiently strong to provide accurate or reliable results. By way ofexample, when blood glucose test strips are calibrated, the test samplemay be capillary blood, while the calibration may be capillary plasma.Further by way of example, when subcutaneous glucose sensors arecalibrated, the test sample may be subcutaneous fluid, while thecalibration sample may be capillary blood.

In another phase, or a second phase, the quality of the raw analytesignal and the calibration measurement data are evaluated to determinewhether to accept or decline a particular data pair for use incalibration. By way of example, dual calibration measurements may bemade, and acceptance may be based upon adequate agreement of the dualmeasurements. Further by way of example, acceptance of the raw analytesignal may be predicated on some feature of that signal, such asmagnitude or variability, for example. In the simplest manifestation ofthis phase of the calibration process, raw analyte signal andcalibration measurement data pairs may be accepted without furtherdiscrimination.

In yet another phase, or a third phase, the raw analyte signal isconverted into an analyte concentration. By way of example, when anelectrochemical glucose sensor is used, a raw current signal (innanoAmperes (nA), for example) may be converted into a glucoseconcentration (in units of mg/dL, for example). A simple way ofperforming this conversion is by simply relating or equating the rawanalyte signal with the calibration measurement, and obtaining aconversion factor (calibration measurement/raw analyte signal), which isoften called the sensitivity. Another simple way of performing thisconversion is by assuming a sensitivity, such as a sensitivity based ona code associated with the measurement sensor, as described above. Thesensitivity may be used to convert subsequent raw analyte signals toanalyte concentration values via simple division ((raw analytesignal)/(sensitivity)=analyte concentration). For example, a raw analytesignal of 10 nA could be associated with a calibration analyteconcentration of 100 mg/dL, and thus, a subsequent raw analyte signal of20 nA could be converted to an analyte concentration of 200 mg/dL, asmay be appropriate for a given analyte, such as glucose, for example.This is often called one-point calibration.

There are many variations of the conversion phase of the calibrationprocess, as will be appreciated. Merely by way of example, thesensitivity can be derived from a simple average of multiple analytesignal/calibration measurement data pairs. Further by way of example,the sensitivity can be derived from a weighted average of multipleanalyte signal/calibration measurement data pairs. Yet further by way ofexample, the sensitivity may be modified based on an empirically derivedweighting factor, or the sensitivity may be modified based on the valueof another measurement, such as temperature. It will be appreciated thatany combination of such approaches, and/or other suitable approaches, iscontemplated herein.

Ideally, the calibration measurement of the first phase described aboveis performed at the time of the analyte sensor is manufactured.Typically, representative sensors from a large batch or “lot” of analytesensors are tested at the site of manufacture, and a calibration code isassigned to the sensor lot. The calibration code may then be used inassociation with the analyte-measuring device to convert the raw analytesignal into an analyte concentration. By way of example, a manufactureror user of the device may enter the code into the device, or a dataprocessor of the device, for such data conversion. Blood glucose teststrips are typically calibrated in this manner, at the site ofmanufacture.

For other types of sensors, including subcutaneous glucose sensors,calibration at the site of manufacture is typically not feasible. Thisinfeasibility may be based on any of a number of factors. Merely by wayof example, variations in the within-lot performance of the analytesensors may be too large, and/or variations in person-to-person responseto a given sensor lot may be too large. When calibration at the site isnot feasible, the calibration measurement must be performed upon fluid,often capillary blood, drawn from or within the wearer of thesubcutaneous sensor. Such a calibration process is often called in vivocalibration.

An example of a calibration process 200 is now described in relation toa flow-chart illustration shown in FIGS. 2A, 2B, and 2C (collectively,FIG. 2). The process 200 comprises the selection 202 of at least onepossible calibration point and the starting 204 of the process with thefirst possible calibration point. Merely by way of example, one mayselect three different calibration points and choose the firstcalibration point in time for further processing, such as a calibrationpoint that is taken within or up to about one hour from the implantationof a measurement sensor, for example.

The first calibration point is then evaluated in at least one of severalpossible processes. For example, the calibration point may be evaluatedas to whether or not (1) a predetermined time has elapsed sinceimplantation or since a prior calibration 206, such as a predeterminedtime of about one hour after implantation, or a predetermined time ofabout 2 hours after a prior calibration, for example; (2) an analyteconcentration (“[G]” in FIG. 2)) associated with the calibration point,such as an analyte concentration from a calibration sensor (for example,from an in vitro measurement of blood from the calibration site) fallswithin a predetermined range 208, such as a predetermined glucoseconcentration range of from about 60 to about 350 mg/dL, for example;(3) a rate of change in analyte concentration from an analyte sensor(for example, from an in vivo measurement of bodily fluid at themeasurement site) since a prior calibration, over a predeterminedperiod, such as about 10 minutes, or about 30 minutes, for example,falls within a predetermined range 210, in any direction (i.e., positiveor negative, up or down), such as a predetermined range for a rate ofchange in glucose concentration change of up to about 2 (mg/dL)/minute,for example; (4) a temperature measurement, such as a measurement ofskin temperature, for example, is within a predetermined range 212, suchas a predetermined range of from about 28° C. to about 37° C., forexample; and/or (5) the sensitivity falls within predetermined limits214, such as within a preset range associated with an analyte sensorproduction lot 216 (for example, a preset range of percentage determinedby a code assigned to a glucose sensor production lot). The evaluationsassociated with the rate of change in analyte concentration and thesensitivity are deemed of particular relevance for applications in whichglucose is the analyte of interest.

If any of the evaluation standards is not met, the calibration point isdeemed unacceptable 218, the next possible calibration point, if any, isselected 220, and that calibration point is then evaluated, as describedabove. If there is no next possible calibration point, the calibrationprocess has failed to provide an acceptable calibration point and ends(not shown). If all of the evaluation standards are met, the calibrationpoint is deemed acceptable 222. If there are more calibration points toevaluate 224, the next possible calibration point is selected 220, andthat calibration point is then evaluated, as described above. If thereare no more calibration points to evaluate 224, the sensitivity factoror factors are calculated 226, in any of a number of ways. Merely by wayof example, an unweighted sensitivity factor (SN), such as the currentfrom an analyte sensor (for example, from an in vivo measurement ofbodily fluid at the measurement site) divided by the analyteconcentration from a calibration sensor (for example, from an in vitromeasurement of blood from the calibration site), may be determined foreach calibration point 228; an adjusted weighting factor (AXM,N), basedon a raw weighing factor (XM,N) and a sensitivity weighing factor (SWF),for example, may be determined for each calibration point 230; and/or aweighted sensitivity (WSN), based on a sensitivity fudge factor (FN),for example, may be determined for each calibration point 232, wherein Nis the number associated with the calibration point (i.e., N=1 for thefirst calibration point 1, N=2 for next calibration point 2, N=3 for thenext calibration point 3, etc.) and M is a number from 1 to N, inclusive(i.e., when N=1, M=1; when N=2, M=1 and M=2, such that there are two rawweighing factors and two adjusted weighting factors; when N=3, M=1, M=2,and M=3, such that there are three raw weighing factors and threeadjusted weighting factors, etc.).

Based on at least one sensitivity factor, the analyte concentrationvalue or values, such as a glucose concentration value, for example, isdetermined 234. By way of example, a raw glucose value (G-raw) may becalculated 236, where the raw glucose value equals the raw analytesignal (I), which may be a current from an analyte sensor, as describedabove, divided by an applicable weighted sensitivity (WS) value. Furtherby way of example, a temperature-compensated glucose value (G-temp) maybe calculated 238, where this value equals the raw glucose value(G-raw), as just described, multiplied by a temperature compensationfactor (TCF) raised to a power equal to the temperature at the timeassociated with the calibration point (T,cal) minus the temperature atthe time associated with the raw analyte signal reading (T,m). Stillfurther by way of example, a lag-compensated glucose value (G-final) maybe calculated 240, where this value equals the temperature-compensatedglucose value (G-temp), as just described, plus a lag factor (k)multiplied by the change in the temperature-compensated glucose value(ΔG-temp) over a period between two acceptable or consecutivecalibration points and divided by the change in time (ΔT) over a periodbetween two acceptable or consecutive calibration points.

The foregoing description provides various calibration or correctionalgorithms that may be used to convert an analyte concentration obtainedfrom bodily fluid to an analyte concentration obtained from blood. Itwill be understood that any of a variety of calibration or correctionprocesses or algorithms may be used, such as any suitable means ordevices described in any of the above-mentioned U.S. Pat. Nos.6,175,752, 6,514,718, 6,565,509, and 6,881,551; U.S. Patent ApplicationPublication No. 2003/0187338 filed Apr. 18, 2003, Schmidtke et al.,Measurement and Modeling of the Transient Difference Between Blood andSubcutaneous Glucose Concentrations in the Rat after Injection ofInsulin, Proc. Of the Nat'l Acad. Of Science, 92, pp. 294-299 (1998);and Quinn et al., Kinetics of Glucose Delivery to Subcutaneous Tissue inRats Measured with 0.3 mm Amperometric Microsensors, Am. J. Physiol.,269 (Endocrinol. Metab. 32), E155-E161 (1995). Once an analyteconcentration is appropriately calibrated, it may be used as a basis forsuitable administration of a suitable amount of a drug, such as insulin,for example, to the patient or subject.

Any of various statistical analyses of the data may follow, such asthose exemplified in the Experimental Study described below, forexample. By way of example, a Clarke error analysis 242 may be conductedto determine values that may be plotted on a Clarke error grid. Suitabledata for such a plot includes analyte concentration values from animplanted analyte sensor and analyte concentration values from venousblood. Further by way of example, root mean square error, average error,slope, intercept, correlation coefficient, and/or the like, may bedetermined 244. Suitable data for such a determination includes analyteconcentration values from an implanted analyte sensor and analyteconcentration values from venous blood. Merely by way of example,analyte concentration values from venous blood (YSI) may be measured ona YSI 2300 instrument (Yellow Springs Instruments, Yellow Springs,Ohio), as described in the Experimental Study that follows. Otherstatistical determinations may be made as desired or useful.

As indicated above, this application is related to, and claims prioritybased on, the Feldman et al. Application, which is the subject of theFeldman et al. Publication. The Feldman et al. Application and theFeldman et al. Publication described Wired Enzyme™ sensing technology(Abbott Diabetes Care) for the continuous measurement of in vivo glucoseconcentrations. Such Wired Enzyme™ sensing technology offers excellentsensor stability, reduced sensor susceptibility to variations in in vivooxygen concentration, and minimized sensor response to commonelectroactive interferents, as demonstrated in the Experimental Studydescribed below.

Experimental Study

In a sensor-response study, 48 subcutaneous sensors based on WiredEnzyme™ sensing technology were implanted in patients with Type 1diabetes (25 in the upper arm, and 23 in the abdomen). These implantedsensors were prospectively calibrated using capillary blood. Whenglucose concentration values from the sensors were compared with thosefrom venous plasma obtained at 15-minute intervals, ninety-eight percentof the values fell in a zone consisting of the clinically accurateClarke error grid zone A and the clinically acceptable zone B. Neitherthe site of the implanted sensor (upper arm versus abdomen) nor the siteof the capillary blood extraction (arm versus finger) affected systemaccuracy. The foregoing study and results are further described herein,following the introduction below.

Introduction

Evidence suggests that improved glycemic control can minimize many ofthe complications associated with Type 1 diabetes. (See, DiabetesControl and Complications Trial Research Group: The Effect of IntensiveTreatment of Diabetes on the Development and Progression of Long-TermComplications in Insulin-Dependent Diabetes Mellitus, N. Engl. J. Med.,329, pp. 977-986 (1993).) Frequent self-monitoring of blood glucose, inconcert with intensive insulin therapy, greatly improves glycemiccontrol.

Continuous glucose sensing provides all of the advantages ofhigh-frequency, discrete testing. It also provides advantages of itsown. By way of example, continuous glucose sensing may provide valuableinformation about the rate and direction of changes in glucose levels,which information may be used predictively or diagnostically. Further byway of example, as continuous glucose sensing occurs at times whendiscrete testing does not usually occur, such as post-prandially orduring sleep, for example, continuous glucose sensing may providesensitive alarms for hyperglycemia and hypoglycemia that may beassociated with post-prandial or resting conditions.

The above-mentioned FreeStyle Navigator® continuous glucose sensor is asubcutaneous, electrochemical sensor, which operates for three days whenimplanted at a site in the body. This sensor is based on theabove-mentioned Wired Enzyme™ sensing technology, a mediatedglucose-sensing technology that offers a number of advantages overconventional oxygen-dependent, electrochemical, glucose-sensingtechnologies, which utilize hydrogen peroxide (H₂O₂) detection at highapplied potential (˜500 mV vs. a silver/silver chloride (Ag/AgCl)reference electrode). (See, Csoregi, E., Schmidtke, D. W., and Heller,A., Design and Optimization of a Selective Subcutaneously ImplantableGlucose Electrode Based on “Wired” Glucose Oxidase, Anal. Chem., 67, pp.1240-1244 (1995).)

Wired Enzyme™ technology works at a relatively gentle oxidizingpotential of +40 mV, using an osmium (Os)-based mediator moleculespecifically designed for low potential operation and stably anchored ina polymeric film for in vivo use. The sensing element is a redox activegel that comprises Os-based mediator molecules, attached by stablebidentate anchors to a polymeric backbone film, and glucose oxidase(GOx) enzyme molecules, permanently coupled together via chemicalcross-linking. This redox active gel is a glucose-sensing gel, whichaccurately transduces glucose concentrations to a measured current overa glucose range of 20-500 mg/dL.

Wired Enzyme™ sensing technology offers three primary advantages overconventional H₂O₂-based detection systems, which rely on oxygen forsignal generation. One advantage is that this Wired Enzyme™ technologyaffords electrochemical responses that are extremely stable. This is notthe case with many other implanted, or in vivo, glucose sensors, whichhave been associated with drifts in sensitivity (output per unit glucoseconcentration) over their lifetimes. (See: Roe, J. N., and Smoller, B.R., Bloodless Glucose Measurements, Crit. Rev. Ther. Drug Carrier Syst.,15, pp. 199-241 (1998); and Wisniewsky, N., Moussy, F., and Reichert, W.M., Characterization of Implantable Biosensor Membrane Biofouling,Fresenius J. Anal. Chem., 366, pp. 611-621 (2000).) Because of thesedrifts, many other implanted glucose sensors require frequent and/orretrospective calibration. By contrast, after an initial break-inperiod, Wired Enzyme™ implanted glucose sensors have extremely stable invivo sensitivities, typically losing no more than 0.1% sensitivity perhour.

Another advantage is that Wired Enzyme™ technology does not rely onoxygen for signal generation. Although oxygen can compete for electronswith the Os-based mediator molecules, and thereby modestly reduce thesensor output, the overall effect is much smaller than exists inconventional H₂O₂-measuring systems, which can generate no signal in theabsence of oxygen. This reduced oxygen dependency results in minimalsensitivity to in vivo oxygen variations and good linearity at highglucose concentrations. Yet another advantage is that Wired Enzyme™implanted glucose sensors operate at an applied potential of only +40mV, which is much gentler than the 500 mV required by H₂O₂-sensingsystems. Oxidation of many interferents (acetaminophen, uric acid, etc.)and subsequent, false, high glucose readings, are minimized at thecomparatively low operating potential of +40 mV associated with WiredEnzyme™ sensors.

The Feldman et al. Application presented preliminary results from anaccuracy study conducted in 30 patients with Type 1 diabetes, usingfrequent venous blood glucose measurements (at 15-min intervals, for 3days), as reference values. The study was performed with a cordedsystem, although use of a wireless system or radio-frequency basedsystem is contemplated according to the present invention. (See: U.S.Pat. Nos. 6,175,752 and 6,565,509 to Say et al. filed on Apr. 30, 1998and Sep. 21, 2000, respectively; and U.S. Patent Application PublicationNo. 2004/0186365 A1 of Jin et al. filed on Dec. 26, 2003.) The study andits results are further described below.

Sensor Description

A continuous glucose sensor 300, as schematically shown in FIG. 3, wasused in the study described above. This continuous glucose sensor 300 isthe FreeStyle Navigator® continuous glucose monitoring device that isbased on Wired Enzyme™ technology, as described above. The sensor 300 isan amperometric sensor that comprises three electrodes, a workingelectrode 302, a reference electrode 304, and a counter electrode 306,contacts of which are shown in FIG. 3. Each of the working electrode 302and the counter electrode 306 is fabricated from carbon. The referenceelectrode 304 is an Ag/AgCl electrode. The sensor 300 has a subcutaneousportion 308 having dimensions of about 5 mm in length, 0.6 mm in width,and 0.25 mm in thickness, as further detailed in the enlarged portion ofFIG. 3.

The working electrode 302 has an active area 310 of about 0.15 mm². Thisactive area 310 is coated with the Wired Enzyme™ sensing layer 312,which is a cross-linked, glucose-transducing gel. As this sensing layeror gel 312 has a relatively hydrophilic interior, glucose moleculessurrounding the subcutaneous portion 308 of the sensor 300 are free todiffuse into and within this glucose-transducing gel. The gel 312 iseffective in the capture of electrons from these glucose molecules andthe transportation of these electrons to the working electrode 302. Aschematic illustration of the Wired Enzyme™ sensing layer 312, showingvarious of its components (as further described below), as well as thepath of electron flow in the direction depicted by arrows 314, from theglucose to the working electrode 302, is shown in FIG. 4A.

The sensing layer or gel 312 comprises a redox polymer mediator 316 ofhigh molecular weight, glucose oxidase (“GOx”) 318, and a bi-functional,short-chain, epoxide cross-linker (not shown), the former two of whichare shown in FIG. 4A. The sensing layer 312 has a mass of 300 ng (at adry thickness of about 2 μm) and comprises about 35% by weight redoxpolymer 316, 40% by weight GOx enzyme 318, and 25% by weightcross-linker. The redox polymer 316, the structure of which isillustrated in FIG. 4B, comprises a modified poly(vinylpyridine)backbone, which is loaded with poly(bi-imidizyl) Os complexes that aresecurely anchored to the backbone via bidentate linkage. (See: U.S.Provisional Patent Application No. 60/165,565 of Mao et al. filed onNov. 15, 1999; U.S. Pat. No. 6,605,200 of Mao et al. filed on Nov. 14,2000; U.S. Pat. No. 6,605,201 of Mao et al. filed on Nov. 14, 2000; U.S.Pat. No. 7,090,756 of Mao et al. filed on Aug. 11, 2003; U.S. Pat. No.6,676,816 of Heller et al. filed on May 9, 2002; and U.S. Pat. No.7,074,308 of Mao et al. filed on Nov. 14, 2003.) This polymer 316 is aneffective mediator or facilitator of electron transport in the sensinglayer.

As shown in FIG. 3, the sensor 300 also comprises an analyte-restrictingmembrane 320, here, a glucose-restricting membrane, disposed over thesensing layer 312. (See: U.S. patent application Ser. No. 10/146,518filed on May 14, 2002 and issued as U.S. Pat. No. 6,932,894.) Themembrane 320 comprises a poly(vinylpyridine)-poly(ethylene glycol)co-polymer of high molecular weight, that is cross-linked using atri-functional, short-chain epoxide. The membrane 320, which is about 50μm thick, serves to reduce glucose diffusion to the active sensing layer312 by a factor of about 50. The hydrophilic membrane 320 provides asurface that is biocompatible, such that bodily irritation from thesubcutaneous portion 308 of the sensor 300 is reduced.

The sensor 300 is associated with an in vivo sensitivity of about 0.1nA/(mg/dL) and a linear response over a glucose concentration range20-500 mg/dL. Additionally, in terms of response to an instantaneouschange in glucose concentration, the sensor 300 is associated with aresponse time of about three minutes.

Sensor Configuration

For each sensor 300 that was used in the study, the subcutaneous portion308 of the sensor was placed into the subcutaneous tissue of the upperarm or the abdomen of a subject or patient using a spring-actuatedinsertion mechanism. (See: U.S. Provisional Patent Application No.60/424,099 of Funderburk et al. filed on Nov. 5, 2002; and U.S. Pat. No.7,381,184 of Funderburk et al. filed on Nov. 5, 2003.) The sensor 300was connected via cord (not shown) to a portable, potentiostat-datalogger device (not shown), which was used to maintain theglucose-sensing working electrode 302 at a potential of +40 mV versusthe Ag/AgCl reference electrode 304, while obtaining and storinginstantaneous current values at 10-second intervals. Each subject wasalso fitted with a small (about 1 cm²), insulated, transdermalskin-temperature sensor, in the immediate vicinity of the continuousglucose sensor 300.

In Vitro Continuous Glucose Sensor Evaluations

In vitro continuous glucose sensor evaluations were carried out at 37°C. in 0.1 M phosphate-buffered saline (PBS) contained in a 2-L jacketedbeaker with gentle stirring. Oxygen dependence experiments wereconducted under two gas mixtures: 95% N₂/5% O₂ and 98% N₂/2% O₂.Interferent evaluations were conducted in separate experiments using 0.2mM acetaminophen, 0.085 mM ascorbate, or 0.5 mM uric acid, also in PBS.In long-term stability experiments, Proclin 500 (Supelco, Bellefonte,Pa.) was added to the interferent evaluation solution at 5 μL/L toretard bacterial growth.

Biocompatibility Testing

Biocompatibility testing was performed on large-scale assembliesconsisting of all sensor components (substrate, electrode inks,membrane, and sensing layer formulations) in proportions correspondingexactly to the actual composition of the continuous glucose sensors 300.(See U.S. Pat. No. 6,175,752 to Say et al.) Cytotoxicity was assessed byISO elution test (minimum essential medium extract) in vitro.Sensitization was assessed with a maximization test (Magnusson Kligmanmethod) in guinea pigs. Irritation was assessed with an ISOintra-cutaneous reactivity test in rabbits. Systemic toxicity wasassessed by a USP systemic injection test in rabbits. Sub-chronicsensitization was assessed by a 30-day implantation test in rabbits.Genotoxicity was assessed by Ames mutagenicity test in vitro.Hemocompatibility was assessed by a hemolysis test (extract method) invitro. All tests were passed.

Clinical Trial Procedure

In a clinical trial, thirty subjects were tested, as described below,over a 3-day trial period. Each subject was fitted with either onecontinuous glucose sensor or two such sensors, and correspondingly, onetransdermal skin temperature sensor or two such sensors, as describedabove. Sensor implant depth was about 5 mm. Each subject was also fittedwith a heparin lock for obtaining venous blood samples. Glucose andtemperature data were obtained at 10-second intervals over the 3-daytrial period, while venous blood samples were obtained at 15-minuteintervals over the trial period. Venous plasma blood glucose values weremeasured on a YSI 2300 (Yellow Springs Instruments, Yellow Springs,Ohio). Capillary blood measurements were also made using theabove-mentioned FreeStyle® blood glucose-monitoring system to enabledevelopment of a prospective calibration algorithm. Arm capillary bloodwas obtained hourly at hours 0-12, 24-30, and 48-54, for all of thesubjects. Finger capillary blood was also obtained at the same times for10 subjects wearing 19 continuous glucose sensors.

Glycemic challenges were performed daily for all subjects. Subjects weregiven intravenous insulin once (0.15 U/kg, followed by 0.10 U/kg ifnecessary to achieve hypoglycemia), and oral glucose (75 g) on twoseparate occasions. Vital signs were monitored at 15-minute intervalsduring administration of intravenous insulin.

An institutional review board approved the trial protocol. Inclusioncriteria for the study were the following: presenting Type 1 diabetes,having a C-peptide concentration of less than 0.5 ng/mL, and being 18years old or older. Thirty subjects were enrolled at three clinicaltrial sites (Renton, Wash.; San Antonio, Tex.; and Walnut Creek,Calif.). Subjects ranged in age from 20 to 85 years, with a mean of 40years. There were eight females and 22 males, comprising three AfricanAmericans, 26 Caucasians, and one Hispanic.

Calibration Procedure

A prospective calibration algorithm was developed in an earlier studyconsisting of 20 sensors (15 arm, 5 abdominal) implanted into subjectswith Type 1 diabetes. The 48 sensors, whose performance is describedhere, were implanted in a separate study conducted sequentiallyfollowing the calibration development set. Therefore, none of the datasets described in the present study was used in development of thecalibration algorithm. For each implant, three capillary blood glucosemeasurements, obtained using the FreeStyle® blood glucose-monitoringsystem, were used as calibration bases, subject to exclusion criteriabased on time, glucose concentration range, rate of glucoseconcentration change, sensitivity, and temperature, as further describedbelow.

As to time, calibration point 1 occurred a minimum of 1 hour afterinsertion, calibration point 2 occurred a minimum of 2 hours after asuccessful calibration point 1, and calibration point 3 occurred aminimum of 21 hours after a successful calibration point 2. As toglucose concentration range, calibration was allowed within a capillaryblood glucose concentration range of 60-350 mg/dL. As to rate of glucoseconcentration change, calibration was restricted to rates of change of 2(mg/dL)/min or less. (A separate study in 20 patients with Type 1diabetes performing normal daily routines (i.e., not performing dailyglucose challenges) showed that the rate of 2 (mg/dL)/min was exceededonly 4% of the time, consistent with other published data. See Jungheim,K., Kapitza, C., Djurhuus, C. B., Wientjes, K. J., and Koschinsky, T.,How Rapid Does Glucose Concentration Change in Daily-Life of Patientswith Type 1 Diabetes?, Abstract, Presented at the Second Annual DiabetesTechnology Meeting, Diabetes Technology Society, Atlanta, Ga. (November2002).) As to sensitivity, calibration was allowed only if the resultingnominal sensitivity (in nA/mM glucose) was within a preset range asdetermined by a code assigned to each continuous glucose sensorproduction lot. As to temperature, calibration was allowed over a skintemperature range of 28-37° C.

The operating sensitivity for the first 2 hours of operation was basedentirely on calibration point 1. However, subsequent operatingsensitivities (after the second calibration point was obtained) werebased on a weighted average of all previously obtained calibrationpoints. This had the effect of refining, and increasing the accuracy of,the calibration as the implant proceeded. This refinement process wasmade possible by the near-negligible drift of the continuous glucosesensor sensitivity with time.

The calibration process also involved a correction for changes in skintemperature underneath the insulated skin temperature probe. Anadjustment of 7% per ° C., relative to the skin temperature at the timeof the operative calibration point, was performed. One sensor (of 49implanted) did not achieve calibration, because of violation of thesensitivity restriction described above. That sensor was excluded fromthe statistical analysis.

Results

The continuous glucose sensor was found to have excellent in vitrostability. This was demonstrated by a plot that showed the responses(current, in nA) of three separate sensors in glucose at 500 mg/dL (inPBS, at 37° C.) versus time (days) over a period of 7 days, as shown inthe Feldman et al. Application and Feldman et al. Publication (see FIG.3). The average total decay in glucose signal over the 7-day test periodwas 1.7%. The mean hourly rate of decay, at 0.011% per hour, isinsignificant. Similar stabilities have been observed in vivo (videinfra).

In vitro testing was also performed to determine the effect of oxygen onthe linearity of the continuous glucose sensors. This results weredisplayed in a plot of the averaged response (current, in nA) versusglucose concentration (mg/dL) of eight continuous glucose sensors thatwere maintained under an oxygen tension of 15 torr, and a plot of thesame, but with the sensors maintained under an oxygen tension of 38torr, as shown in the Feldman et al. Application and Feldman et al.Publication (see FIG. 4). (The lowered O₂ levels reflect the reducedlevels found in subcutaneous tissue. See Burtis, C. A., and Ashwood, E.R., eds., Tietz Textbook of Clinical Chemistry, W.B. Saunders Co.,Philadelphia, Pa. (1999).) Curves drawn for the two plots exhibitexcellent linearity (R²=0.9999 for both curves) over the glucose rangeof from 18 to 540 mg/dL. The curves differ in slope by only 4%, withdifferences varying from 0.4% at 36 mg/dL to 3.5% at 540 mg/dL. Theseresults indicate that the continuous glucose sensors are only minimallyoxygen dependent.

In vitro testing was performed to determine the effect of threeinterferents, namely, acetaminophen, ascorbate, and uric acid, at thetop of their normal physiological or therapeutic range (0.2 mM, 0.085mM, and 0.5 mM, respectively (see Burtis, C. A., and Ashwood, E. R.,eds., Tietz Textbook of Clinical Chemistry, W.B. Saunders Co.,Philadelphia, Pa. (1999)), on continuous glucose sensors. Theglucose-equivalent interferences were 3 mg/dL for acetaminophen, 19mg/dL for ascorbate, and 3 mg/dL for uric acid, tested at these levels.The interferences due to uric acid and acetaminophen areinconsequential, which can be attributed largely to the low operatingpotential (+40 mV versus Ag/AgCl) associated with the continuous glucosesensors.

In vivo testing of continuous glucose sensors, as implanted, wasperformed. Representative results of the testing are shown in FIG. 5, inthe form of an overlay plot of representative data from an abdominallyimplanted continuous glucose sensor (current (in nA) versus time (indays)) and venous plasma glucose values (glucose concentration (inmg/dL) versus time (in days)). It should be noted that the data wereraw, that is, not calibrated and not corrected for temperature, and notime-shifting of the data was performed.

The results are noteworthy in that they demonstrate what is obviously anexcellent correlation between the raw current values associated with thecontinuous glucose sensor and the venous plasma glucose concentrations.No substantial lag between subcutaneous and venous glucoseconcentrations is evident. The results are also noteworthy in that theydemonstrate that the sensitivity of the implanted continuous glucosesensor is essentially unchanged over the 3-day implantation period.Given this stability in signal sensitivity, it is possible to schedulethree calibration points in the first 24 hours of the implantation, withno additional calibration points during the final 48 hours.Additionally, given nearly negligible sensor drift, it is possible touse a weighted average of multiple calibration points as a basis foraccounting for the operating sensitivity of the implanted sensor. Suchuse of a weighted average is helpful reducing any error inherent in thecapillary blood glucose measurement that is used for calibration.

In vivo testing of continuous glucose sensors, as implanted in the armsof the subjects, was performed. Representative results of the testingare shown in FIG. 6 in the form of a plot (glucose concentration (inmg/dL) versus time (in days)) of representative data from anarm-implanted continuous glucose sensor (one of the 48 calibratedsensors), venous plasma, and capillary blood from an arm-stick. Itshould be noted that the current data obtained from the arm-implantedcontinuous glucose sensor was converted to glucose concentration data,by way of a prospective calibration that was based on the arm-capillaryblood measurements that were obtained using the FreeStyle® bloodglucose-monitoring system. No time-shifting of the data was performed.

The results are noteworthy in that they demonstrate an excellentcorrelation between subcutaneous and venous plasma glucose values, whichis indicative of both reliable sensor function and accurate calibration.As noted above, the representative data set shown in FIG. 6 wascalibrated using arm-capillary blood measurements. The results are alsonoteworthy in that no significant change in accuracy was found (videinfra) when the data were calibrated using finger-capillary bloodmeasurements.

In vivo testing of continuous glucose sensors, as simultaneouslyimplanted in the arm and in the abdomen of a single subject, wasperformed. Representative results of the testing are shown in FIG. 7, inthe form of a plot (glucose concentration (in mg/dL) versus time (indays)) of representative data from an arm-implanted continuous glucosesensor, an abdomen-implanted continuous glucose sensor, and venousplasma. The results demonstrate good agreement between the glucosevalues measured at subcutaneous sites in the arm and in the abdomen.

These results also demonstrate good agreement between the subcutaneousglucose values associated with the arm and abdomen and those associatedwith the venous plasma, although some deviations from the latter wereobserved on the first night of implantation, when the subcutaneousvalues fell intermittently below the venous plasma values. Based on data(not shown) for spatially adjacent sensors implanted at a single site,it is believed that these deviations result from interactions betweenthe sensor and the insertion site, not from systematic differencesbetween venous and subcutaneous glucose in the body. The deviations arevirtually always negative (that is, the glucose value from the implantedcontinuous glucose sensor is lower than the glucose value from thevenous plasma) and tend to occur at night and early in the course of the3-day implantation.

The cause of the negative deviations described above is unknown,although some possible causes may be put forward, as follows. It may bethat cells or other subcutaneous structures adhere to the sensorsurface, blocking glucose ingress. It may be that blood clots form uponsensor insertion, exerting a similar glucose-blocking effect. (Bloodclots were not observed to adhere to the active areas of explantedsensors (that is, sensors that were removed from the body afterimplantation), but that does not preclude their presence prior toexplantation.) It may be that constriction of local blood vessels, dueto external pressure effects, restrict glucose delivery to the sensorsite.

It should be noted that the deviations described above are not frequent.Sensitivity was reduced by 40% or more for only 4% of the roughly 3,500sensor-hours represented by this study. Overall, the system performedwell, as demonstrated by statistical data described below.

A Clarke error grid of data (glucose concentration from the continuousglucose sensor versus that in venous plasma (mg/dL)) from all of the 48continuous glucose sensors (25 in the arm and 23 in the abdomen) thatwere inserted in the 30 subjects, is shown in FIG. 8. These data wereprospectively calibrated, with no time-shifting, using arm-capillaryblood data. The grid represents 12,667 data pairs. Approximately 98% ofthe data fall within a zone consisting of the clinically accurate “A”region and the clinically acceptable “B” region of the Clarke errorgrid.

A tabular summary of statistical data from the Clarke error grid andfrom the implanted continuous glucose sensors is presented in Table 1below. In Table 1, the data are categorized according to theimplantation site, either arm or abdomen, and/or the calibration site,either arm or finger.

TABLE 1 Summary of Statistical Data Calibration Clarke error grid ARESubset Description Site N^(a) % A % B % C % D % E (%) A All sensors Arm12,667 67.9 29.7 1.2 1.1 0.0 17.3 (25 arm, 23 abdominal) B 25 sensorsArm 6,656 67.0 30.3 1.8 1.0 0.0 17.7 (arm) C 23 sensors Arm 6,011 69.029.1 0.6 1.3 0.0 17.2 (abdominal) D^(b) 19 sensors Arm 4,987 67.7 29.31.8 1.1 0.0 17.4 (arm, finger calibration available) E^(b) 19 sensorsFinger 4,922 68.2 29.8 1.1 0.8 0.0 17.0 (arm, finger calibrationavailable) ^(a)Number of continuous sensor/venous plasma data pairs.^(b)Subsets D and E have slightly different n values, since there weresmall variations in the time at which calibrated operation (and hencemeaningful venous/subcutaneous glucose pairs) began.

More particularly, the data described above in relation to FIG. 8appears in Table 1 in association with a Subset A, representingarm-based calibration, for the data from the 48 continuous glucosesensors (25 in the arm and 23 in the abdomen). This data is furtherbroken down in Table 1 for the 25 sensors that were implanted in the arm(Subset B) and the 23 sensors that were implanted in the abdomen (SubsetC). The data demonstrates that when arm-capillary blood calibration wasemployed, there was no significant difference between the use of aninsertion site in the arm, associated with an absolute relative error(“ARE”) of 17.7% (for 25 sensors, Subset B), and use of an insertionsite in the abdomen, associated with an ARE of 17.2% (for 23 sensors,Subset C).

The other data appearing in Table 1 were obtained from 19 sensors thatwere used to simultaneously determine glucose values using calibrationsamples withdrawn from both the arm (Subset D) and the finger (Subset E)of a subject on an hourly basis for hours 0-12, 24-30, and 48-54. Thedata were obtained in this manner from 10 subjects. The data show thatof 4,987 continuous sensor/venous plasma data pairs in Subset D,representing arm-based calibration, 67.7% were found to be in region Aof the Clarke error grid, 29.3% in region B, 1.8% in region C, 1.1% inregion D, and 0.0% in region E. The data further show that of the 4,922continuous sensor/venous plasma data pairs in Subset E, representingfinger-based calibration, 68.2% were found to be in region A of theClarke error grid, 29.8% in region B, 1.1% in region C, 0.8% in regionD, and 0.0% in region E. The data in Table 1 demonstrate that there isno significant difference between arm-capillary blood calibration(ARE=17.4%) and finger-capillary blood calibration (ARE=17.0%).Accordingly, arm-capillary blood may be used more or less as effectivelyas finger-capillary blood as the basis for one-point in vivocalibration.

Conclusions

All of the continuous glucose sensor data presented above (with theexception of the raw data overlay of FIG. 5) were derived using aprospective calibration based on nominal calibration times of 1, 3, and24 hours after implantation. The calibration algorithm was developedusing a separate data set for 20 similar implanted continuous glucosesensors. None of the data reported here was used in development of thecalibration algorithm.

As demonstrated herein, the continuous glucose sensor used in the studyis extremely stable in terms of in vivo sensitivity after a modestacclimation process (during which sensitivity may rise by a few percent)that is generally complete in a few hours. Because sensor output is sostable, calibration points may be concentrated in the first 24 hours ofuse and calibration may be periodically or continuously refined asmultiple calibration points are obtained. Both of these strategies maybe advantageous for a number of reasons. By way of example, theconcentration of calibration points in an early portion of theimplantation period, such as the first 24 hours, for example, may beadvantageous in that no calibration is required over the remainingportion of the implantation period, such as the final 48 hours of a72-hour period of implantation, for example. Further by way of example,either this concentration of calibration points early on, or theabove-described refinement of the calibration, as opposed to the use ofthe most recent calibration point as a basis for calibrating the sensor,or both, may be advantageous in the reduction or minimization ofcalibration error.

It is noteworthy that no time-shifting of data was used in the studydescribed herein. That is, all of the data are real-time data.Time-shifting of data has been used frequently in the literature tocompensate for any error associated with physiological time lags betweenthe subcutaneous and reference glucose measurements or associated withslow system response times. As it is believed that time-shifting ofglucose values and prospective calibration are incompatible concepts,time-shifting of data, such as glucose values, may be avoided accordingto the present invention.

Based on the statistical data provided herein, the average physiologicaltime lag (subcutaneous-venous) associated with the continuous glucosesensors tested was found to be about 8 minutes. This value wasdetermined by the theoretical exercise of finding the minimum inabsolute relative error as reference and subcutaneous values weretime-shifted. Of this 8-minute lag, about 3 minutes and 5 minutes can beattributed to the response time of the sensor, and to physiology,respectively. In a recent review (see Roe, J. N., and Smoller, B. R.,Bloodless Glucose Measurements, Crit. Rev. Ther. Drug Carrier Syst., 15,pp. 199-241 (1998)) of various subcutaneous glucose measurementstrategies, lag times ranging from 2 to 30 min, with an average lag of8-10 minutes, were reported, which is in good agreement with thefindings of this Experimental Study. A more complete study ofphysiological glucose lags based on the raw data of this study has beenpresented at the 39^(th) Annual Meeting of the German DiabetesAssociation, in Hannover, Germany, May 19 to May 22, 2004, by Feldmen,B., and Sharp, C., under the title, Correlation of GlucoseConcentrations in Intersitital Fluid and Venous Blood during Periods ofRapid Glucose Change.

The data for the continuous glucose sensor tested, as shown in FIGS.5-7, demonstrate excellent linearity at both high and low glucose valuesinduced by glycemic challenges. The continuous glucose sensor faithfullytracks in vivo glucose values over the physiologically relevant range.Overall, for the complete data set, 98% of readings fall within a zonethat consists of the clinically accurate Clarke error grid zone A andthe clinically acceptable zone B, as shown in FIG. 8 and Table 1. Thisrepresents excellent performance. It should be noted that no only doesthe continuous glucose sensor perform outstandingly, it providesdirectional trend information, a very desirable predictive or diagnostictool.

The data summarized in Table 1 demonstrates that there was nosignificant difference between arm-capillary blood calibration,associated with an ARE of 17.4%, and finger-capillary blood calibration,associated with an ARE of 17.0%. Thus, arm-capillary blood served as analmost equally accurate, and less painful, calibration tool, relative tofinger-capillary blood. While not studied here, it is contemplated thatrubbing of skin adjacent to a calibration site (see the FreeStyle® BloodGlucose Testing System, Test Strip Package Insert, TheraSense, Inc.,Alameda, Calif. (2000)), such as a calibration site in the arm, mayimprove the efficacy of the capillary blood from that site as acalibration tool. The data summarized in Table 1 also demonstrates thatwhen arm-capillary blood calibration was employed, there was nosignificant difference between the use of an insertion site in the arm,associated with a ARE of 17.7% (for 25 sensors), and use of an insertionsite in the abdomen, associated with a ARE of 17.2% (for 23 sensors).

The possibility of a large variation between arm- and finger-capillaryblood values has been put forth in various studies conducted under theextreme conditions of glucose loading, followed by intravenous deliveryof insulin. (See Koschinsky, T., and Jungheim, K., Risk Detection Delayof Fast Glucose Changes by Glucose Monitoring at the Arm, Diabetes Care,24, pp. 1303-1304 (2001).) In fact, under normal use conditions, thesedifferences are not significant unless glucose is changing very rapidly.(See Bennion, N., Christensen, N. K., and McGarraugh, G., Alternate SiteGlucose Testing: A Crossover Design, Diabetes Technol. Ther., 4, pp.25-33 (2002).) Restriction of calibration to rates of less than 2 mg/dLper min virtually eliminates this possible source of error.

The present invention is applicable to corded or cabled glucose-sensingsystems, as described above, as well as other analyte-sensing orglucose-sensing systems. For example, it is contemplated that suitableresults, along the lines of those described herein, may be obtainedusing a wireless glucose-sensing system that comprises a pager-sized,hand-held, informational display module, such as a FreeStyle Navigator®wireless glucose-sensing system. The FreeStyle Navigator® systememployed herein is capable of providing real-time glucose information at1-minute intervals and information regarding rates and trends associatedwith changes in glucose levels. This system is further capable ofproviding a visual indication of glucose level rates, allowing users todiscriminate among glucose rate changes of less than 1 mg/dL per minute,1-2 mg/dL per minute (moderate change), and greater than 2 mg/dL perminute (rapid change). It is contemplated that sensors having featuressuch as these will be advantageous in bringing information of predictiveor diagnostic utility to users. The FreeStyle Navigator® system is alsodesigned to provide hypoglycemic and hyperglycemic alarms withuser-settable thresholds.

Each of the various references, presentations, publications, provisionaland/or non-provisional United States patent applications, United Statespatents, non-U.S. patent applications, and/or non-U.S. patents that havebeen identified herein, is incorporated herein in its entirety by thisreference.

Other aspects, advantages, and modifications within the scope of theinvention will be apparent to those skilled in the art to which theinvention pertains. Various modifications, processes, as well asnumerous structures to which the present invention may be applicablewill be readily apparent to those of skill in the art to which thepresent invention is directed upon review of the specification. Variousaspects and features of the present invention may have been explained ordescribed in relation to understandings, beliefs, theories, underlyingassumptions, and/or working or prophetic examples, although it will beunderstood that the invention is not bound to any particularunderstanding, belief, theory, underlying assumption, and/or working orprophetic example. Although various aspects and features of the presentinvention may have been described largely with respect to applications,or more specifically, medical applications, involving diabetic humans,it will be understood that such aspects and features also relate to anyof a variety of applications involving non-diabetic humans and any andall other animals. Further, although various aspects and features of thepresent invention may have been described largely with respect toapplications involving partially implanted sensors, such astranscutaneous or subcutaneous sensors, it will be understood that suchaspects and features also relate to any of a variety of sensors that aresuitable for use in connection with the body of an animal or a human,such as those suitable for use as fully implanted in the body of ananimal or a human. Finally, although the various aspects and features ofthe present invention have been described with respect to variousembodiments and specific examples herein, all of which may be made orcarried out conventionally, it will be understood that the invention isentitled to protection within the full scope of the appended claims.

1. A method, comprising: subcutaneously positioning at least a portionof an in vivo glucose sensor; obtaining a signal from the positionedsensor; qualifying the obtained signal to determine a sensitivity factoraccording to a first predetermined criteria; obtaining a calibrationmeasurement from a calibration sensor from an off-finger calibrationsite; qualifying the obtained calibration measurement to determine thesensitivity factor based on a second predetermined criteria; determiningthe sensitivity factor using the qualified signal from the in vivoglucose sensor and the qualified calibration measurement from thecalibration sensor; and determining a suitability of the sensitivityfactor for calibration of the in vivo glucose sensor by evaluatingwhether the sensitivity factor is within a predetermined range assignedto a production lot of the in vivo glucose sensor.
 2. The method ofclaim 1, further comprising calibrating the in vivo glucose sensor onlyif the sensitivity factor is determined to be within the predeterminedrange.
 3. The method of claim 1, further comprising disabling acalibration routine to calibrate the in vivo glucose sensor if thesensitivity is determined to not be within the predetermined range. 4.The method of claim 1, further comprising: determining an averagesensitivity factor by averaging a plurality of data pairs, each datapair including the obtained signal from the sensor and the calibrationmeasurement; and calibrating the in vivo glucose sensor based on theaverage sensitivity factor.
 5. The method of claim 4, wherein theaverage sensitivity factor is derived from a weighted average of theplurality of data pairs.
 6. The method of claim 5, wherein the weightedaverage is a weighted average of all previously obtained data pairs. 7.The method of claim 1, wherein a sample for the calibration measurementis less than about 1 microliter.
 8. The method of claim 1, wherein asample for the calibration measurement is less than about 0.5microliters.
 9. The method of claim 1, wherein a sample for thecalibration measurement is less than about 0.2 microliters.
 10. Themethod of claim 1, wherein the calibration site is an arm.
 11. Themethod of claim 1, wherein the calibration site is an abdomen.
 12. Themethod of claim 1, wherein the calibration sensor is an in vitro sensor.13. The method of claim 1, wherein the calibration sensor is an in vivocalibration sensor.
 14. The method of claim 1, further comprisingevaluating the calibration measurement for calibration acceptability.15. The method of claim 14, wherein the evaluating the calibrationmeasurement for calibration acceptability comprises evaluating whetherthe calibration measurement is within a predetermined range of analyteconcentration.
 16. The method of claim 15, wherein the predeterminedrange comprises about 60 mg/dL to about 350 mg/dL.